Unity中Update与FixedUpdate：输入触发物理操作的合理性疑问

Great question—let’s break this down clearly and put your mind at ease!

First off: Your approach is totally valid and actually recommended for this specific use case. Here’s why:

• Update runs every frame, which aligns perfectly with input handling. Instant input events like Input.GetMouseButtonUp(0) (a single frame where the mouse is released) can easily be missed if you check them in FixedUpdate. FixedUpdate runs at a fixed time interval (default 0.02s), so if the release happens between two FixedUpdate calls, your code would never catch it. Update ensures you don’t miss these critical one-off input events.

• The key rule to remember is: Don’t run continuous physics updates in Update, but one-time, trigger-based physics actions (like adding force/torque on a mouse click) are completely safe.

Let’s look at your code example:

void Update() { 
    if (Input.GetMouseButtonUp(0)) { 
        ball.GetComponent<Rigidbody>().AddForce(vector); 
        ball.GetComponent<Rigidbody>().AddTorque(vector2); 
    } 
}

This is totally fine! You’re only modifying the Rigidbody once, in response to a single input event. Since it’s not running every frame, there’s no risk of physics instability from fluctuating frame rates.

A small optimization tip: Cache the Rigidbody reference in Start() instead of calling GetComponent every frame (it’s a minor performance hit, but good practice for cleaner, faster code):

private Rigidbody _ballRb;

void Start() {
    _ballRb = ball.GetComponent<Rigidbody>();
}

void Update() {
    if (Input.GetMouseButtonUp(0)) {
        _ballRb.AddForce(vector);
        _ballRb.AddTorque(vector2);
    }
}

To recap the core guidelines:

• Use Update for all input handling (especially one-off events like clicks/releases) to avoid missing them.
• One-time physics triggers (like your mouse-release force) can safely live in Update.
• For continuous physics changes (e.g., holding a key to apply constant force), check input state in Update, then perform the physics action in FixedUpdate (using Time.fixedDeltaTime if needed to keep consistency across frame rates).

内容的提问来源于stack exchange，提问作者Wouter Vandenputte